Using wavelet analysis to detect changes in water temperature regimes at multiple scales: effects of multi‐purpose dams in the Willamette River basin

Abstract

Maintaining the natural complexity of water temperature regimes is a key to maintaining diverse biological communities. Insect communities, food webs, and fish respond to the magnitude and duration of water temperature fluctuations. Disruption of these natural patterns has the potential to alter physiological processes, behavioural adaptations, and community structure and dynamics. We analysed multiple >300-day time series of water temperature from the Willamette River basin, Oregon, to assess the impact of large multi-purpose dams on water temperature variability at temporal scales ranging from 1 to 32 days, short temporal scales that are commonly ignored. We applied wavelet analysis to quantify the variability of water temperature at multiple temporal scales simultaneously. We compared water temperature regimes above and below dams and before and after dam construction. The advantages of wavelet analysis are the ability to examine all temporal scales simultaneously and independently as well as the ability to preserve the temporal context of the wavelet coefficients. We were able to detect significant ( p < 0.0001) reductions in water temperature variability, defined as the variability of the wavelet coefficients, as a result of dams at the 1-, 2-, 4-, and 8-day scales. There were no significant differences in water temperature variability between managed and natural flows at the 16-and 32-day scale ( p ¼ 0.80). In addition to the well-documented effects of dams on seasonal patterns in water temperature or on water temperature extremes, our results demonstrate that dams have significantly muted the small temporal scale variance in water temperature patterns to which many organisms may have been adapted. Conserving or restoring natural temperature patterns in rivers will require attention to these small-scale complexities.The independent variable was the variability of wavelet coefficients at each of six scales, calculated independently for each of 22 times series of daily water temperature data.

Beyond the mean: The role of variability in predicting ecological effects of stream temperature on salmon

“…The insufficiency of a simplifying model or rule of thumb is not necessarily worthy of interest; however, this simplifying model has led us to ignore a critical element of riverine thermal regimes, temporal variability, which is simultaneously being altered v www.esajournals.org through multiple anthropogenic activities including dam-building (Steel and Lange 2007) and climate change (Webb andNoblis 1995, Schär et al 2004). We have demonstrated that, in fish, thermal regimes which might appear similar with respect to mean temperature or total degree days may produce important differences in emergence timing and/or physiological status at emergence.…”

“…Climate change, dams, irrigation, and landscape conversion are all agents of changing stream and river thermal regimes that can lead to altered variance (Webb and Noblis 1995, Schär et al 2004, Steel and Lange 2007. It is well-understood both that increased water temperatures speed up metabolic rates of aquatic organisms and, therefore, alter the timing of life-history transitions such as egg hatching and fry emergence and that there are lethal thermal thresholds for many species.…”

Understanding the Temperature Variations and Thermal Structure of a Subtropical Deep River-Run Reservoir before and after Impoundment

“…At the same time, they can change and influence the aquatic environment and ecology of the river valley where they are built. Specifically, water temperature, which is critical to aquatic flora and fauna [1,2], changes considerably after a reservoir is impounded [3][4][5][6][7][8][9][10]. In addition, the temperature in the downstream reach of a reservoir varies due to the impacts from the releases of this reservoir [3,[5][6][7][8][9][10][11][12][13][14][15].…”

Simulation of river water temperatures during various hydro-peaking regimes

“…Hydro-peaking may cause similar rapid and frequent changes to the water temperature, known as thermo-peaking (Toffolon et al 2010). Zolezzi et al (2011) have derived methods for identifying thermo-peaking events, while Steel & Lange (2007) have described the scale, variability, and recurrence of thermo-peaking. The ecological responses to rapid changes in water temperature are, however, not thoroughly studied and understood, with exceptions such as Bruno et al (2013) that studied the effect on the benthic community.…”